Back in late 2012, Pentax introduced a new multi-layer lens coating dubbed HD, for "High Definition". According to its maker, the HD coating offered a couple of key benefits, but it's not until recently that we've been able to make a real-world comparison to test these claims. We've now done so, and the results are rather interesting.

When it launched the HD coating, Pentax promised that it would be more durable, yield better color balance and better control reflections, a key to reducing lens flare. The company predicted a better than 50% reduction in reflections across the entire visible spectrum, so we were led to expect a pretty significant improvement -- but at the time there was no way to make a direct comparison against a non-HD coated lens. More recently, though, Pentax has also updated its entire DA Limited lens lineup with the new HD coating, and as it happens, I own a couple of the original, non-HD versions of the same lenses. A comparison was clearly in order, then.

Shortly before departing on vacation to Hong Kong, I tried a few different tests, shooting my own non-HD glass side-by-side with the new HD variants using our review sample of the Pentax K-3 body. First of all, I shot some images right near sunset in downtown Knoxville, Tennessee, intentionally provoking lens flare by keeping the sun fairly close to the image frame. I also took some shots of nearby brick buildings with a vast array of rooftop airconditioners, providing lots of fine detail. And then I took camera and lenses back home for some bokeh testing, given that the newer lenses are also said to have rounder aperture blades.

Pentax's overhauled DA Limited lineup includes a 20-40mm f/2.8-4 lens, the first in the family to feature either weather-sealing or an optical zoom.

While I was at it, since we had the entire HD lens lineup -- including the new HD Pentax DA 20-40mm F2.8-4 ED Limited DC WR, the company's first Limited lens with optical zoom or weather sealing -- I also took similar shots with every other optic in the family. (For the 20-40mm lens, I repeated each shot at wide angle and telephoto.)

You can find the results from all six lenses in the gallery of our Pentax K-3 review. Reduced resolution, side-by-side comparisons of the two lenses I own (the smc PENTAX DA 21mm F3.2 AL Limited and smc PENTAX DA 40mm F2.8 Limited) against their newer HD-coated equivalents follow below. I didn't note significant difference in terms of detail or bokeh -- perhaps a very slight reduction in sharpness, but despite focusing in live view in live view mode with contrast detection, focus isn't in precisely the same place, so it's hard to call. As you can see, though, there's a very worthwhile improvement in flare for both of these lenses.

Admittedly, this was a rather extreme test, and intentionally so. Were I to buy either lens today, though, I think I'd definitely look to pick up the HD versions, even if the improvement probably isn't enough to make me sell my existing non-HD lenses and trade up. See the Pentax K-3 gallery for the full-resolution images.